1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) mobile phone in a digital communications system and, more particularly, to a circuit for eliminating external interference signals in such a mobile phone.
2. Description of the Related Art
Generally, a digital communications system may be designed to employ CDMA and an advanced mobile phone service (AMPS) which employs frequency modulation. The CDMA cellular system is usually used in dual mode to allow reception of an analog signal. Namely, the system may be used not only in CDMA mode but also in AMPS mode. However, in this case, when communicating in digital signals, there often occurs analog interfering signals which cut a call signal or cause an error in the received digital signals. This error is called intermodulation distortion (IMD), which affects a CDMA mobile phone near an AMPS radio station. A strong external interference signal overloads the low noise amplifier of the CDMA phone so that the IMD component strongly interferes with the CDMA frequency band, causing it to be off-line. An exemplary description of the influence of such interference signals will now be provided.
The received interference signals pass through the low noise amplifier of a conventional CDMA phone to produce second and third order harmonics Eo, which may be expressed by Eq. (1) as: EQU Eo=k.sub.1 Ei+k.sub.2 Ei.sup.2 +k.sub.3 Ei.sup.3 (1)
If there are generated dual tones with the same amplitude and different frequencies, the dual tones are expressed by Eq. (2) as: EQU Ei=A(cos .omega..sub.1 t+cos .omega..sub.2 t) (2)
such that Eo may be further expressed by Eq. (3) as: ##EQU1##
As expressed by Eq. (3), two signals with different frequencies F1 and F2 are amplified to generate non-linear components in the form of the products obtained by multiplying the mixed forms of the second and third orders. The second order mixing is 2F1, 2F2, F1+F2 and F1-F2. Such signals of the mixed component occur outside the narrow band of most systems and, therefore, are negligible. The third order mixing is 2F1+F2, 2F1-F2, 2F2-F1 and 2F2+F1 whose signals cause IMD to occur in the pass band of most systems.